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Chemiosmotic Coupling - Molecular Biochemistry I Oxidative...

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Oxidative Phosphorylation Copyright © 1999-2007 by Joyce J. Diwan. All rights reserved. Molecular Biochemistry I
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w Respiration-linked H+ pumping out of the matrix conserves some of the free energy of spontaneous e - transfers as potential energy of an electrochemical H+ matrix inner membrane outer membrane inter- membrane space mitochondrion cristae w Conventional view of mitochondrial structure is at right. w Respiratory chain is in cristae of the inner membrane . w Spontaneous electron transfer through respiratory chain complexes I, III & IV is coupled to H+ ejection from the matrix to the intermembrane space. Because the outer membrane contains large channels , these protons may equilibrate with the cytosol.
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3-D reconstructions based on electron micrographs of isolated mitochondria taken with a large depth of field, at different tilt angles have indicated that the infoldings of the inner mitochondrial membrane are variable in shape and are connected to the periphery and to each other by narrow tubular regions. matrix inner membrane outer membrane inter- membrane space mitochondrion cristae
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between the lumen of cristae & the intermembrane space . There is evidence also that protons pumped out of the matrix spread along the anionic membrane surface and only slowly equilibrate with the surrounding bulk phase, maximizing the effective H+ gradient . Electron micrograph by Dr. C. Mannella of a Neurospora mitochondrion in a frozen sample in the absence of fixatives or stains that might alter appearance of internal structures. Wadsworth Center website . Tubular cristae connect to the inner membrane via narrow passageways that may limit the rate of H+ equilibration
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A total of 10 H+ are ejected from the mitochondrial matrix per 2 e - transferred from NADH to oxygen via the respiratory chain. The H+/e - ratio for each respiratory chain complex will be discussed separately. Matrix H + + NADH NAD + + 2H + 2H + + ½ O 2 H 2 O 2 e - – – I Q III IV + + 4H + 4H + 2H + Intermembrane Space cyt c Spontaneous electron flow through each of complexes I, III, & IV is coupled to H+ ejection from the matrix.
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Complex I ( NADH Dehydrogenase) transports 4H+ out of the mitochondrial matrix per 2e - transferred from NADH to CoQ. Matrix H + + NADH NAD + + 2H + 2H + + ½ O 2 H 2 O 2 e - – – I Q III IV + + 4H + 4H + 2H + Intermembrane Space cyt c
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Lack of high-resolution structural information for the membrane domain of complex I has hindered elucidation of the mechanism of H+ transport. Direct coupling of transmembrane H+ flux & e - transfer is unlikely , because the electron-tranferring prosthetic groups, FMN & Fe-S, are all in the peripheral domain of complex I. Thus is assumed that protein conformational changes are involved in H+ transport , as with an ion pump. inner mitochondrial membrane matrix NAD + NADH Complex I FMN peripheral domain membrane domain FMN A B FMN Peripheral domain of a bacterial Complex I membrane domain PDB 2FUG N2
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Complex III (bc1 complex): H+ transport in complex III involves coenzyme Q (CoQ).
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